碩士 / 國立成功大學 / 地球科學系專班 / 96 / Uranium- and thorium-series have a series of radioactive elements with different half lives. Because the parent nuclides (238U, 234U) have higher solubility than the daughter radionuclides (234Th, 230Th), when ground water flows through the rocks, the uranium isotopes are preferentially transported in natural waters, such as in rivers, lakes, seas. Meanwhile α-recoil effect would cause 234U/238U activity ratio in waters to be greater than 1. Organisms who inhabit natural waters, such as diatoms, corals, and foraminifera, may uptake the uranium isotopes into their tests. These organisms incorporate relatively few daughter nuclides (234Th, 230Th) due to their low solubility in natural waters, thus causing the parent-daughter radioactive disequilibrium. The disequilibrium is used not only to determine the age of biological fossils, but also provide a tool for studying the geological processes (e.g., weathering) and environmental changes. In this thesis, I studied the distribution of uranium and thorium isotopes and their biogeochemical behavior in diatom frustules. The potential use of U and Th isotopes in diatom frustules as a tool for geochronological dating and for paleo- environmental studies will be discussed.
This study shows that while pure and integral diatom frustules can be separated from sediments by heavy-liquid and sodium hexametaphosphate separation method, a chemical cleaning is needed in order to remove the contaminants from the diatom frustules. Various cleaning and dissolution methods are tested in this study. I found that diatom frustules can be dissolved with HF or NaOH solution, however the HF dissolution is not favorable as it can also easily dissolve the detrital materials that may have been coated on the diatom frustules. The use of NaOH dissolution can selectively dissolve diatom frustules rather than the ditrital minerals. The contaminants on diatom frustules can be cleaned effectively by using HNO3 and/or HCl + NH2OH•HCl solutions, but not by H2O2, NaF, and/or NH2OH in HAc solutions. Saturated KMnO4 solution, combined with 6 N HCl, can also be effective in cleaning, although the cleaning solution may contain higher uranium background.
This study shows that the U and Th isotope compositions in diatom frustules are completely different from those in detrital contaminants. In pure diatom frustules contain uranium in the range of 24 ppb to 61 ppb. In contrast to those in lithogenic detritus, pure diatom frustules also contain higher ratios of 238U/232Th and 230Th/232Th and their 234U/238ratios are usually greater than 1 (e.g., close to their seawater ratio for marine diatoms). These features suggests that diatom frustules incorporate U isotopes , rather than Th isotopes, from the natural waters, providing a new tool for geochronological, paleoceanographical, and paleoenvironmental studies.
| Identifer | oai:union.ndltd.org:TW/096NCKU5135009 |
| Creators | Yi-Chen Shiu, 徐意淳 |
| Contributors | Shang-De Luo, 羅尚德 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 51 |
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